For the frequent appearing sub-synchronous oscillation（SSO）phenomena of sending terminal grid-connected for wind farm in wind power collection areas

the equivalent model of interconnecting multiple direct drive permanent magnet wind turbine generators（D-PMSGs）to the grid is established and the mechanism of SSO is analyzed by the equivalent model for D-PMSGs in this paper.Then the wind farm

converter station etc the related sites are incorporated into the monitoring system based on synchronized phasor measurement unit（PMU）tool.The subsynchronous oscillation is judged by detecting the active power oscillation waveform of the tie line and combining with the sub-synchronous harmonic frequency characteristics of the contact line.The simulation results show that the risk of sub synchronous oscillation existed when the D-PMSG wind farms are connected to the grid.The monitoring and recording results of sub-synchronous harmonics monitoring device and other torsional vibration monitoring protection device are basically the same

then it proves the feasibility of the monitoring system.The construction and application of the monitoring system will provide reference for the follow-up study of the mechanism of synchronous oscillation and suppression measures.